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\begin{document}

\IBVShead{4682}{10 March 1999}


\IBVStitle{A STUDY OF THE MICROVARIABLE STAR V1674 CYGNI}

\IBVSauth{V.P. GORANSKIJ$^1$, E.A. KARITSKAYA$^2$, K.N. GRANKIN$^3$, O.V. EZHKOVA$^3$}

\IBVSinst{Sternberg Astronomical Institute, Universitetskii prospect, 13,
Moscow, 119899, Russia.\\
e-mail: goray@sai.msu.su}

\IBVSinst{Institute for Astronomy of the Russian Academy of Sciences,
Pyatnitskaya, 48, Moscow, 109017, Russia.\\
e-mail: karitsk@sai.msu.su}

\IBVSinst{Astronomical Institute, Astronomicheskaya, 33, Tashkent, 700052,
Uzbekistan.\\
e-mail: grankin@silk.glas.apc.org; ezhik@luck.silk.org}

\SIMBADobj{V1674 Cyg}

\begintext

V1674 Cyg (BD\,+34\deg3816) was discovered as a microvariable star by
Walker and Quintanilla (1978) (WQ) during their study of the X-ray
binary Cyg X-1 (V1357 Cyg), which is located 53\arcs\ South.  The star
had shown both long time non-periodic variations (in the range of
10\fmm555--10\fmm570 B), and periodic variations with the components
P$_1$ = 1\fday3608 (A$_1$ = 0\fmm0037) and P$_2$ = 0\fday8055 (A$_2$ =
0\fmm0032).  Karitskaya and Goranskij (1996) confirm the variability in
the range of 10\fmm012--10\fmm035 V with one of two possible periods,
P$_1$ = 4\fday223 or P$_2$ = 1\fday3009.  Lyuty (1972), R\"ossiger and
Luthardt (1989) considered this star to be constant, and selected it as
a comparison star for their photometry of Cyg X-1. Some observers use
this star for comparison purposes due to its colour close to that of
Cyg X-1, or due to lack of other available stars in small CCD fields.
So the knowledge of photometric behaviour of V1674 Cyg will be useful
to treat the earlier observations of Cyg X-1.

We monitored V1674 Cyg in the course of our photometric study of Cyg
X-1 during four seasons in 1995--1998 (JD 2450009--51040). The
observations were carried out with the 100 cm reflector of Tien-Shan
observatory (Kazakhstan) using the four-channel WBVR photometer with
dichroic beam-splitters (Kornilov, 1998), and with the 48 cm reflector
of Mt.\ Maidanak observatory (Uzbekistan) using a single-channel UBVR
photometer.  The comparison and check stars were BD\,+34\deg3812 (A2)
and BD\,+35\deg3895 (A0).  A total of 581 BVR observations were taken in
89 nights.  Additionally we have U and instrumental ultraviolet (w)
band measurements.  A small systematic difference between the two data
sets was found in the mean magnitudes, and eliminated.  The
observations have the accuracy equal to 3--5 mmag in each of B, V and R
bands (three nights with lower accuracy were averaged and also used in
the calculations). The frequency analysis in B, V and R bands shows, in
general, similar results, because the colour changes are almost within
detection limits. So here we show only `smoothed B band measurements'
calculated with the formula B$_s$ = (B + V + R + $\langle$B -- V$\rangle$ + $\langle$B $-$
R$\rangle$)${} /3$, to decrease the noise in the set.

\IBVSfig{10cm}{4682-f1.eps}{The amplitude spectra of the initial set
(a); of the residual set after prewhitening for P$_1$ (b); of the
residual set after prewhitening for P$_1$ and P$_4$ (c); spectral
window (d).}

\vskip-11pt
Figure 1a shows the amplitude spectrum of V1674 Cyg which covers the
range of periods between 0\fday1 and 1000\day.
The spectrum resembles that calculated by WQ after
eliminating the non-periodic
variations in WQ data. Both spectra have double peaks,
their height being 30 per cent larger in our spectrum.  Undoubtedly,
the star is a periodic variable.  We made a digital experiment
described by Terebizh (1992) to estimate the statistical significance
of the peaks in the spectrum.  For each time of an observational data
point, we chose an accidential magnitude from the list of observations.
So we mixed the list of star magnitudes for the given list of times of
observations. The mixed chaotic set preserves the distributions of
magnitudes and times of the initial set.  The horizontal line marks the
highest amplitude level of 1.8 mmag  for the chaotic set's spectrum
achieved in 10$^5$ realizations of light curves with the trial periods,
so the probability of accidental appearance of peak with this amplitude
is less than $10^{-5}$. The largest peaks of the real set have the
amplitude of 5.4 mmag. So they are reliable with a probability more
than 99.999 per cent.

The periods of the three predominating peaks are the following:
P$_1$ = 4\fday272$\pm$0\fday002,
P$_2$ = 1\fday3007$\pm$0\fday0001,
and P$_3$ = 0\fday80513$\pm$0\fday00005.
The last one was noted by WQ.
These three periods are aliases interconnected with a window
feature of 1 c/day (Fig.~1d).
The periodicities belong to V1674 Cyg -- the power spectrum
of the check star does not show any details.

Figure 1b shows the amplitude spectrum of the residual set
after prewhitening the initial set for P$_1$ =4\fday272.
The probability level of  $10^{-5}$ has also been calculated.
The peaks of the residual spectrum are highly reliable:
P$_4$ = 5\fday267$\pm$0\fday002; P$_5$ = 6\fday676$\pm$0\fday002;
P$_6$ = 1\fday2300$\pm$0\fday0001; P$_7$ = 0\fday83839$\pm$0\fday00005,
the highest peak being 3.6 mmag.

\IBVSfig{10cm}{4682-f2.eps}{
Left: the light curves of the three predominant
periods P$_1$--P$_3$ (the secondary wave with P$_4$ is eliminated).
Right:  the light curves of secondary periods (the primary wave with
P$_1$ eliminated).}

\vskip-12pt
Repetition of the prewhitening procedure for the larger amplitude
wave with P$_4$ leads to a residual set with a flat amplitude
spectrum (Fig.~1c). This test shows that the
periods P$_4$ -- P$_7$ are
interdependent, and reflect only one wave in the light
curve. So, our analysis detects only two independent waves in
V1674 Cyg, with the full amplitudes of 10.8 and 7.2 mmag. Periods
P$_1$ = 4\fday272 and P$_4$ = 5\fday267 may be preferable only because of their
higher peak amplitudes, but their aliases and double values
are also possible.

The flat spectrum of the residual set and the absence of low-frequency noise
in our data suggest that the trends in our observations
are small relative to those found by WQ in B band.
We find marginal evidence of a trend with the amplitude of
0\fmm012 in the second season of our observations in JD 2450360--50399.
We have also analysed the influence of possible season-to-season trends on
our results and found it to be insignificant.
Analysing HIPPARCOS photometry of $\eta$ Cyg (NSV 12586, Sp.\ K0III),
a comparison star used by WQ, we reject
the assumption that this star may be the source of any trends or
periodicities.

The principal results in the individual photometric bands are given in Table 1 (the number of observations, the total range of variability,
the mean magnitude, the dispersion, and the full amplitudes of
harmonic component are given).

\begin{table}
\begin{center}
{\normalsize Table 1}
\vskip2mm
\begin{tabular}{ccr@{\ --\ }lcccc}
\hline
Band & N & \multicolumn{2}{c}{Total range} & Mean & Dispersion &  Ampl.\ P$_1$& Ampl.\ P$_4$\\
     & (obs.)          &  \multicolumn{2}{c}{(mag)} & (mag)&  (mmag)    &  (mmag)        &  (mmag)        \\
\hline
U    &  75     &10.71 & 10.79& 10.741& 17.2     &   ---     & ---  \\
B    & 596     &10.58 & 10.64& 10.601& 8.9      &  12.2     &  7.0\\
V    & 598     & 9.98 & 10.03& 10.001& 8.4      &  12.8     &  8.2\\
R    & 589     & 9.43 & 9.49 & 9.456 & 8.9      &  11.2     &  7.0\\
\hline
\end{tabular}
\end{center}
\end{table}

V1674 Cyg is a double star.
Our CCD astrometry and photometry for the faint
companion gives: $\rho$ = 5\farcs9; $\theta$ =170\deg;
B = $14\fmm0\pm0.2$; V = $13\fmm0\pm0.1$; R = $12\fmm0\pm0.1$
(JD 2451199.15).
For the bright component: B = 10\fmm65; V = 10\fmm07; R = 9\fmm57.
(The summary brightnesses are presented in the Table). The colours agree
with the primary's F8III spectrum (Bregman et al., 1973).

In spite of the periodic nature of the light curve, we cannot yet find a
unique type for V1674 Cyg. One can suggest that the bright
component is a pulsating variable [the ratio P$_1$/P$_4$ = 0.81 is close to
that between the 2nd and the 1st overtones of mixed mode
pulsators like CO Aur or GSC 4018.1807 (Antipin, 1997)].
The star may be also a spotted, or an ellipsoidal variable.
Spectroscopic study may help to
choose a real type for V1674 Cyg. Besides,
the variability may also be due to the faint companion.
Then its amplitude may be about 0\fmm19 V, with EW type.

Taking into account recent results of the OGLE project (see, e.g.,
Udalski et al., 1994),  we can conclude
that the low-amplitude periodic variable stars with the periods of few days
are not unique, but widely spread.
They are found in so-called `miscellaneous' group. So the classification
of OGLE variables will face us with the same problem as that of V1674 Cyg.
GCVS has many types for poorly studied irregular variables
(i.e.\ I, IA, IB, L, LB, S:), but unfortunately it does not have
any type for poorly studied regular, or periodic, or multiperiodic ones.
Without the special type definition, V1674 Cyg should stay in the GCVS as an
unique variable with the symbol (*) in the column ``Type''.

\smallskip
This study was supported by Russian Ministry of Science through
the budget item `Optical Monitoring of Unique Astrophysical Objects',
and partly by Russian Foundation for Basic Research by Grant
No.\,98-06-17067. V.P.G. and E.A.K. are thankful to K.O.~Kuratov and
the staff of Tien-Shan Observatory for hospitality.

\references
Antipin, S., 1997, {\it Astron.\ Astrophys.}, {\bf 326}, L1

Bregman, J., Butler D., Kemper, E., et al., 1973, {\it Lick Obs.\ Bull.}, No.\,647.

Karitskaya, E.A., Goranskij, V.P., 1996, {\it IBVS}, No.\,4404

Kornilov, V.G., 1998, {\it Baltic Astronomy}, {\bf 7}, 513

Lyuty, V.M., 1972, {\it Variable Stars}, {\bf 18}, 417

R\"ossiger, S., Luthardt, R., 1989, {\it Mitt.\ Ver\"and.\ Sterne}, {\bf 11}, 177, (H. 8.)

Terebizh V.Yu., 1992, Time series analysis in astrophysics, Moscow: Nauka

Udalski, A., Kubiak, M., Szymanski, M., et al., 1994, {\it Acta Astron.},
{\bf 44}, 317

Walker, E.N., Quintanilla, A.R., 1978, {\it Monthly Not.\ Roy.\ Astron.\ Soc.},
{\bf 182}, 315

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